The objective of this project is the study of the changes occurring in cardiac and fast and slow muscle cell constituents in the course of development and aging. Adult mammalian muscles of different functional types show well defined differences in their physiological, biochemical and histochemical characteristics. Our first objective is to explore the differentiation in embryonic and post-partum developmental stages and to investigate what changes occur in muscles of aged animals. The proposed experiments will also deal with various aspects of the determination and maintenance of the above distinct muscle types in terms of changes in activity pattern, neural influence, development and aging. BIBLIOGRAPHIC REFERENCES: Roy, R.K., Sreter, F.A. and Sarkar, S., Tropomyosin (TM) subunits in Avian and Mammalian Muscles: Differences in Gene Expression in Adult and Embryonic Fibers, Fed. Proc. 36, 625, 1977. Rubinstein, N., Sreter, F., Mabuchi, K., Pepe, F. and Gergely, J., Use of Type Specific Antimyosins to Demonstrate the Transformation of Individual Fibers in Chronically Stimulated Fast Muscle, Fed. Proc. 36, 584, 1977.